Countershaft transmissions having a plurality of speed ratios in each direction of operation are particularly useful in the drive line of heavy duty vehicles such as wheel loaders, rubber tired log skidders, and lift trucks. Such transmissions are advantageous in that a plurality of rotating clutches and associated continually meshing gears can be so positioned on the usual parallel shafts as to allow considerable design flexibility and an adaptability to the elevational drop requirement between the input and output axes.
One of the problems with such countershaft transmissions is that they have required too many shafts or gears. For example, many of them have an extra idler gear in order to provide the reversing function. This requires an additional bearing system along another axis parallel to that of the primary shafts at increased expense.
Another problem with these prior countershaft transmissions is that they have been axially too long. One reason for this is that some of them have incorporated two juxtaposed rotating clutches and two gears along the output shaft axis, so that the added coupling member required to releasably couple the output shaft to the vehicle drive line adds excessive length at that point. Still other transmissions have included housing structures with internal walls or rotating clutches located axially beyond the walls, and both of these constructions have undesirable overall lengths.
Accordingly, what is needed is a structurally simple, and both radially and axially compact countershaft transmission providing at least two forward and two reverse speeds using a maximum of three shafts, a minimum number of gears, and four rotating clutches. Preferably, the construction should provide room on the output shaft for a releasable coupling member without unduly contributing to the transmission's overall length at that location.
Moreover, the desired countershaft transmission should preferably maximize parts commonality by using similarly sized rotating clutch elements, and have a long service life by designing the elements and their connections such that undesirably high operating speeds or clutch plate engagement speeds are avoided. And, the shafts, gears and rotating clutches should be so arranged as to fit axially between the front and rear walls of a separable housing in order to allow convenient assemby and disassembly thereof.